1. Field of the Invention
This invention is in the field of adaptive optics systems and more particularly relates to apparatus for sensing wavefront distortions in a return beam of light.
2. Description of the Prior Art
It is known that a light beam can be significantly distorted as it passes through the atmosphere. This problem, for example, has limited the resolution of images received by telescopes of stellar bodies deep in space. In addition, atmosphereic distortion has posed a severe restriction on attempts to irradiate objects with laser beams when those objects are located great distances from the emitting laser. Other distortions present in practical optical systems also add to reduce the resolution of systems both for viewing an object or irradiating the object.
To overcome distortions of these types, it has been proposed that adaptive optical systems be employed. Such systems are designed to sense wavefront distortions and to compensate for them by adding compensating modifications in the outgoing laser beam wavefront, in the case where the object is being irradiated, or by adding compensations to the optical path when viewing an object.
The system concepts for adaptive optical systems used to irradiate an object currently fall into two general categories. These are: (1) outgoing wave modulated systems; and (2) return wave wavefront measurement systems such as those employing shearing interferometers.
Outgoing wave systems tag a wavefront by modulating the outgoing laser beam. The object irradiance due to the arrival of the radiation from a given subaperture of the transmitting telescope may be tracked by detecting a reflected return with this differentiating modulation. Irradiance is maximized by making wavefront adjustment on the subaperture to maximize the tagged radiation received back from the irradiated object.
Return wave systems use light returned from an irradiated object to measure aberrations in the optical path between the object and focal plane of the optical system. By making these measurements on the return wavefront, as received by the same optical aperture that transmitted the radiation beam, the aberrations at each subaperture can be determined and appropriate adjustments made in the optical path of the subaperture to compensate exactly for these errors. Such return wave systems require that the measuring system and laser share the identical optical paths so that measurements correspond precisely with the laser wavefront passing through a particular subaperture of the transmitter optics. In addition, return wave systems can use the light emanating from an object to improve the image of that object when viewed through the same optical system used to make the wavefront measurement.
Adaptive optical systems which have been described in the patent literature include those by O'Meara in U.S. Pat. Nos. 3,731,103; 3,975,629; 3,979,585; 3,988,608; and 4,016,415. Another is the real-time wavefront correction system described by Hardy in U.S. Pat. No. 3,923,400.
Hartmann wavefront sensors have been proposed for use in adaptive optical systems of return wave type. Conventional Hartmann wavefront sensors employ a detector array of photosensitive cells, each of which samples a portion of the light entering an aperture of an optical system. The subaperture portions are focused by a lenslet array onto the individual photosensitive cells which detect the position of the focused subaperture beam portions, and are indicative of the wavefront tilt of the optical beam.
Hartmann-type sensors are unique compared to other systems, such as those employing a chopper wheel because they collect and sample virtually 100% of the light entering the optical system. Additionally, the wavefront tilt over the subapertures can be measured even when the phase of the light from one side to the other exceeds two waves. The systems are limited only by the size of the cell detectors. Hartmann-type sensors can also detect wavefront tilts in non-coherent sources of white light sources because they are independent of wavelength and only depend on the angle of tilt of the wavefront. Additionally, Hartmann-type sensors are well suited to adaptive optical systems since they measure tilt angles of wavefronts, and not optical phase differences. This tilt angle is exactly what is needed to compensate for optical path errors independent of wavelength.
Despite these unique advantages, Hartmann-type sensors have not been widely accepted for use in adaptive optical systems because they have heretofor suffered from significant disadvantages. For example, they have normally been used for static (d.c.) measurements of the centroid position of each subaperture. D.C. centroid detectors are relatively noisy and produce a very low signal-to-noise ratio for measurements of wavefronts emitted from weak sources. In addition, a major disadvantage of previously designed Hartmann-type sensors was the requirement for extremely accurate and stable optical-mechanical alignment. Without this, such systems could not accurately detect the position of subaperture centroids reproducibly. This was a severe handicap for systems requiring fast, dynamic measurements of wavefront distortion. In such systems, it was virtually impossible to maintain accurate optical-mechanical positioning in practical operational environments.